For a long time now I have been, shall we say, gently encouraged by two of my friends to write about a subject that is both familiar, and yet also strangely alien and poorly understood. Sleep behaviour. We still know comparatively little about this subject: not only about the big stuff like its function, but even about its distribution within animals. I am not, by the way, about to tackle the big questions about sleep, nor am I going to discuss the different types of sleep (e.g., REM vs NREM sleep) and on how they differ from creature to creature. Instead I’m interested in the more superficial stuff, like how and where animals sleep, and on the postures they adopt.
What we do know about sleep is scattered widely in the literature and it isn’t easy to obtain any sort of review of the subject. There is at least one book devoted to it though: Maurice Burton’s Sleep and Hibernation in the Animal World (Odhams Books, 1969), one of twelve books written for the Animal World in Colour series. Burton is well known for amassing large amounts of anecdotal data on the natural world (he also wrote books covering cryptozoology and the emotional lives of animals), and while the books in the Animal World in Colour series include a lot of often bizarre and fascinating information, they are completely unreferenced and at least some of the content looks apocryphal and difficult to be confident about. Despite these problems I have borrowed heavily from his book on sleep here.

Needless to say, I’m only going to be looking here at sleep in tetrapods, though where relevant I will mention other animals. The first thing to mention is that some animals get by without virtually any sleep, or in fact may even go without sleep entirely. Among vertebrates, all identified non-sleeping species are fish (Kavanau 1998). The species concerned have little in common (they include blind cave-dwellers, various sharks including some triakids, lamnids, hammerheads and sevengills, and mackerels and scombrids), but all have lifestyles where ‘visual input is greatly reduced or absent during lengthy periods’, where ‘schooling greatly reduces needs for sensory information, particularly visual’, or where the animals lead a ‘comparatively routine existence in essentially featureless, open waters’ (Kavanau 1998, p. 269) [sleeping cow from here].

Perhaps because no tetrapod lives in this way, it seems that all indulge in at least some sleep activity. But some sleep very little, with one of the best examples being seabirds: some species are on the wing for weeks at a time and appear to be constantly active. It has been suggested that they might sleep on the wing, but it’s considered equally plausible that they simply delay their sleep. And having mentioned ‘sleep on the wing’, while it’s ‘well known’ that swifts sleep in flight, the evidence for this (which essentially consists of observations of swifts seen decreasing in altitude during the early hours of the morning) is poor and we still require detailed studies (Rattenborg 2006).

Some ruminants also seem to engage in very little sleep. This might be because they need to remain constantly alert for predators, but it might also be because their biology requires a virtually continuous cycle of eating and ruminating. Burton (1969) wrote of observations made on goats which suggest that, during a rest period that lasts about 8 hours, they never really relax fully, and it is even stated that some ruminants (like cattle) sleep with their eyes open (this must be true: before indulging in the nefarious activity known as cow-tipping you’re supposed to shine a torch into the cow’s eyes to see whether it reacts or not. No reaction = asleep). Ruminants do sleep with their eyes closed at least sometimes however, as verified by the sleeping cow shown above. Passive stay apparatus (special locking mechanisms in the limbs) mean that some hoofed mammals can remain standing virtually indefinitely, and such mammals (which include deer, rhinos and horses) are therefore capable of sleeping while standing. Having said that, they will still lie down to rest when they want to. I photographed the donkey shown here as I initially thought it was dead.

It used to be thought that the biggest extant hoofed mammals – the giraffes – did not sleep at all, but this was shown to be incorrect by Grzimek (1956) who demonstrated that adult giraffes slept recumbently on the ground for short periods (2.5-6 minutes), resting the head on the hindquarters or ground and holding the neck in an arced posture. This posture is not unique to giraffes but is also practised by okapis and some bovids. The sleep behaviour of giraffes was looked at in detail by Tobler & Schwierin (1996) who showed that giraffes slept both while standing and while recumbent for a total of 4.6 hours per 24 hours.

Evidently little known is that elephants lie down on the ground to sleep properly: even today it seems widely thought that elephants ‘crush themselves under their own weight’ if they lie down, but this is just not true. In the wild, elephants sleep for 1-4.5 hours (per 24) in a recumbent position, with individuals showing preferences for sleeping on their left or right sides. Tobler (1992) reported that captive Asian elephants slept in a recumbent position for an average of 72 minutes a night. Elephants have been photographed propping themselves up against termite mounds and other structures when lying down to sleep, and in captivity they have been reported to construct ‘pillows’ from straw. The photo at the top shows a sleeping captive elephant (from wikipedia): for images of African elephants sleeping recumbently in the wild go here.

Large birds are also now known to recline on the ground for at least short periods. Burton (1969) reported how a scientist at Frankfurt Zoo monitored the sleep behaviour of ostriches: for 7-8 hours per night, they slept lightly with the neck erect. But for an average of 9 minutes each night they would lie on their sides, stretch their necks and legs out on the ground, and sleep deeply, during which time they could not be roused by lights or noises (except the very loudest noises). Emus, incidentally, are said to sleep deeply with the head and neck resting on the back, rather than lying on the ground.

Moving to other birds, terrestrial, ground-foraging birds like pheasants roost on the ground or in the low branches of trees, and simply huddle up and tuck in their appendages. I have been interested to learn, however, that small passerines (and perhaps others) don’t sit out on branches during the night, but secrete themselves into small spaces. I know this because, while creeping through the woods at night once (don’t ask), I discovered a Blue tit Cyanistes caeruleus tucked deep within a crevice in the bark of a tree, only its tail tip sticking out. It seems that most small birds climb deep into foliage, into crevices or cavities to sleep, and don’t sit out in the open. Remember this next time you unnecessarily tear down the ivy from a tree or fence. Some passerines, including wrens, weavers and bush-tits, form large huddles within cavities (Perrins 1976). Burton (1969) wrote that birds which climb vertically on substrates, like nuthatches and treecreepers, also sleep in vertical postures, with nuthatches resting head-down and treecreepers resting head-up. Woodpeckers are also reported to sleep clinging vertically to vertical trunks (Perrins 1976). A few passerine species have been reported sleeping on their nests even when the nests were empty of eggs.

According to Burton (1969), birds do not put their heads under their wings when they sleep, but instead bury the bill into the scapular feathers. He also stated that penguins are the only birds that properly hide the bill under the wing: they can’t submerge any part of their head within their scapular feathers, because their feathers are so short.
I wonder what the hesperornithines (which lacked long feathers and possessed strongly reduced wings) did [adjacent King penguin photo by J. P. Matth from here].

So far I haven’t mentioned the fact that some animals sleep with only ‘half of the brain': this is called unihemispherical sleep, and it allows the animal to stay partly alert while in a restful state. Animals capable of this may be able to keep one eye open and alert for danger (as in, for example, wading birds that rest while exposed in open habitats like mudflats), or may be able to regulate movement while also resting. The latter ability is important to marine mammals, which doze at or near the water surface and still need to regulate their breathing. Cetaceans sleep unihemispherically, and a number of pinnipeds are also reported to sleep floating in the water, hanging in a vertical posture in the case of Grey seals Halichoerus grypus, Northern fur seals Callorhinus ursinus and walruses Odobenus rosmarus. Weddell seals Leptonychotes weddelli are reported to do this while submerged at their breathing holes in the ice. It has also been noted that some pinnipeds sleep underwater in cetacean fashion, ‘rising periodically, while still asleep apparently, to take a breath and then sinking again to continue their slumber’ (Burton 1969, p. 27). This is correct, as verified by this photo (below) of a Harbour seal Phoca vitulina sleeping underwater (from Rock Paper Lizard – thanks to Vasha for the heads-up).

This has been nothing like a thorough review of the subject and, as I said at the start, it’s been very superficial and I haven’t looked at the big questions about sleep behaviour. For tetrapods at least, sleep would appear to be essential, even if members of some lineages can get by on very little. Speaking of which, at the time of writing it’s gone 2 am. Good night.

Comments

Totally fascinating and, as far as I know, all correct information. It is funny how sleep researchers rarely pay attention to stuff like sleep position, as opposed to EEG data, sleep duration, timing and patterns.

I work at a wildlife rehab center in the US and at night we have to catch songbirds in their aviaries to be released the next day. Once it gets dark they go sleep, usually concealed in foliage. Wrens can be very hard to find. Jays and thrushes perch up high and are conspicuous. With just a little light you can catch them easily by hand. Small birds will cluster and you can catch 4-5 with one hand. If you miss they quickly come to life and can only be caught with a net.

On the subject of small passerines sleeping in samll crevices, I think its come for blue tits etc to roost in nest boxes in groups up to 10 or 20, huddling together to preserves heat on cold winter nights

Very interesting little overview. Especially with the pictures. Though I realize it’s an unscentific reaction — the giraffe sleep posture looks very uncomfortable.

I’m kind of interested in hooved animals’ ability to sleep while standing. As well as a bat’s ability to sleep hanging upside down (which you didn’t mention), I’d be curious to know a little about how those strategies evolved.

I always wonder how my frog-eyed gecko sleeps without eyelids. The only way you can tell he’s asleep is that his legs are sprawled out and his pupils are impossibly thin. I suppose that’s akin to closing one’s eyes–if you close off the pupil, you’re achieving much the same result as closing your eyelids.

My ferret curls up in a tight ball, even though hes so long. (he never flops out of one side like you see cats and dogs do) He sleeps up to 20 hours a day, and is active at random times.

I cant sleep at night unless I spend some time on my belly. sometimes I sleep in this position, sometimes not. Im told the optimal positions on my back, but I dont like it, and how many people do sleep that way??

I cant sleep at night unless I spend some time on my belly. sometimes I sleep in this position, sometimes not. Im told the optimal positions on my back, but I dont like it, and how many people do sleep that way??

I can only fall asleep on my belly. Except when I’m very tired; in that case I can fall asleep on my back — but then I always soon get a nightmare and wake up! I do occasionally turn around, but falling asleep only works well on the belly.

Sleeping on a side is impossible — leads to squeezing the blood off in the lower shoulder.

What’s so uncomfortable about lying on my back is that I have to lift all inner organs to breathe in. I’d need gastralia to prevent abdominal collapse. :o) (Wait — that can’t be it. Crocodiles have gastralia and faint when turned on their backs.)

Two probable reasons:
– People with Asperger’s “syndrome” like pressure. I have a few symptoms, and I like pressure on the belly.
– When I was a baby, all doctors believed laying babies on the belly was best, so that’s what was done with me. Later, laying babies on the back became fashionable (apparently there’s even a connection between lying on the belly and sudden infant death); and indeed, like the rest of the family, my little sister much prefers lying on her back…

“I can only fall asleep on my belly. Except when I’m very tired; in that case I can fall asleep on my back — but then I always soon get a nightmare and wake up!”

This is weird, I ususally can’t sleep on my back for the same reason. I tend to have nightmares in this position, I don’t know why. My mom was one of those people who believed that babies should sleep on their stomachs. She thought we would end up with flat heads if we slept on our backs…Seriously.

I have a friend who is a very committed vegetarian. If we eat out together I usually avoid ordering meat just to prevent an arguement. She grew up on a farm and told me that whenever you see a cow or a horse lying in a field it means that they’re sick. According to her, it’s unnatural for these animals to sleep lying down. I know she’s wrong but I wonder if this is a common belief among vegetarians? I usually don’t bother discussing it because she lived on a farm and I didn’t so therefore she knows more then me. But still…Has anyone else heard of this before?
I also wanted to extend my condolences to Darren, I was sorry to read about your loss in a previous post.

There is a good deal of information about the roosting (and other) habits of Blue Tits and Great Tits in “Birds as Individuals” by Len Howard.

In somewhat the tradition of Walking With Dinosaurs, I think there should be a show that examines the sleeping behavior and posture of birds, and it should be titled Sleeping With Great Tits (And Other Nocturnal Adventures With Various Birds).

@Susan: I have no idea about your question, but why should it have anything to do with her being vegetarian? I think that belief would much more likely to be connected with growing up on a farm than with being a vegetarian.

I think what Susan means is that people who adopt vegetarianism for primarily emotional reasons may have a tendency to assert any odd factoid they may have come across that seems to support the idea that primates can’t digest meat properly, or that hunting is cruel and unsustainable, or that farmed animals are all sick and miserable. And so on. I’m not taking issue with the generalisations, just noting the prevalence of confirmation bias in those committed to a position one way or the other, as on any question.
Growing up on a farm makes it likely this person has seen farm animals sleeping lying down (but probably not often), and is also familiar with the same animals sleeping while standing. Also, some farm animals get sick, and many people understandably feel icky about the idea of eating sick animals. Makes sense to err on the side of caution, not to eat animals that sleep lying down? But there’s only one way to be sure…

@John: Makes sense. I’d think it’s definitely plausible that she doesn’t eat meat because she grew up on a farm and got attached to the animals (or, conversely, saw firsthand how dirty and disgusting they can be). But I was just pointing out that, since we didn’t know precisely WHY she was a vegetarian (plenty of people do it for perceived health benefits, independent of any feelings toward the animals), we can’t necessarily see how that would affect her ideas about animals.

Large seals sleep during deep descending dives, and while ashore sleep in apnea (breath hold). Anadder blog had a bit on sleep today too: Anadder
where I commented that sleep is “anti-photosynthetic” or so. Eyelids are primarily sleep/burrowing related, I’d guess, as opposed to dust / foreign-object protection.

I wonder if the pupillary light reflex of the oculomotor cranial nerve is derived from primarily sleep or from focusing, I’d guess sleep came first.

Possible correlation to the photic (solar powered) sneeze and dark adaptation in the trigeminal cranial nerve in humans?

This thread has reached a new low for TetZoo. I am very disappointed in those people who posted disparaging speculations on why certain people have chosen not to eat other animals.

Vegetarians have many different, often entirely rational reasons for their choice. You cannot evaluate how rational their choice is without determining how rational is your own, a task that you cannot (by definition) do with any objectivity.

“If we weren’t meant to eat other people, why do they taste like pork?”

On the opposite side, many primates sleep very long. Wild gibbons fall asleep long before dusk. I also seen chimps and monkeys in a zoo sleeping already in afternoon.

Apparently, ground squirrels who hibernate and periodically wake up for short periods, also have episodes of normal sleep between hibernation.

Off-topics, I cannot fall asleep without turning a few times to lay on left and right side. I guess it is some innate behavior for stretching muscles. I also find very interesting how people pick beds in a room. When you have several people in a dormitory, always the most distant (and hidden) places are picked first. I suppose, its about picking most safe place.

Do any tetrapods other than humans ever have sleep apnea? Specifically, the kind induced by malformations of the back of the palate, smooth muscle in the back of the throat closing inappropriately, etc.? Even any other primates? It seems like this would be the kind of thing natural selection would eliminate…except in humans, apparently (says the sufferer of said condition)…

Seriously, it does appear on three other pages, but with no more attribution. I recall it from decades ago, but cannot guess where. Variations that Google finds are “The poor man”, “rich man”, and “fool” for “businessman”.

This is weird, I ususally can’t sleep on my back for the same reason. I tend to have nightmares in this position, I don’t know why.

I’m the same. My guess is that in that position my airflow eventually gets blocked. If I’m dreaming, the fear that arises from being unable to breathe turns the dream into a nightmare. I distinctly recall certain dreams going from benign to terrifying right before waking up with a snort.

My pet conure sleeps in a little tent in her cage, with her tail sticking out. She’ll also nap on a branch with one leg up, but that’s usually only for short periods of time. Her preference by far is to fall asleep on my leg with my hand over her like a blanket. Terribly difficult to get any work done when she wants to nap.

Its a reference to Laurence Lambe’s restoration and description of Gorgosaurus. i.e. lying straight out on the stomach with the legs folded parallel to the body.

Google turns up a sketch by John Sibbick, and there’s a partial quote out there about “hunger impelled wanderings”, but I’m sure there was a fuller quote in Adrian Desmond’s “Hot-Blooded Dinosaurs”. Which of course I don’t have to hand.

Neat post. Too bad there was no reptile love in the sleep survey, but then I’d imagine herp related sleep studies are probably few and far between in the literature (though I think I remember reading about unihemispherical sleep existing in sea turtles).

My guess is that in that position my airflow eventually gets blocked. If I’m dreaming, the fear that arises from being unable to breathe turns the dream into a nightmare. I distinctly recall certain dreams going from benign to terrifying right before waking up with a snort.

That is definitely not the case in my case (er… yeah). When my nose closes due to excess production (I’m allergic to dust mites), and I happen to be dreaming, I dream that my nose closes; it doesn’t get reinterpreted, it enters the dream directly. And when I have a cold, I often try to sleep on my back because that way it’s easiest for liquids to leave the nasal cavity. Tends to work.

I tried sleeping on my front last night (at 03-40, the earliest I could get to bed). I cannot do it, have to be on my side. Well done Dave (Godfrey)… the ‘recumbent gorgosaur’ thing refers to Lawrence Lambe’s inferred resting posture for Gorgosaurus (depicted in Fig. 48d in Lambe (1917)), later re-drawn by John Sibbick. Lambe wrote ‘This position of rest, and particularly the recumbent one of repose at full length were probably those [sic] most frequently assumed by a reptile having the form, and the supposed sluggish disposition of Gorgosaurus‘ (p. 83). Yes, whenever I look at a tyrannosaur I think ‘sluggish’ (I don’t: that was a joke).

Incidentally, my hard-copy of Lambe (1917) is copied from the personal copy that once belonged to Alick Walker. It’s covered with annotations where he compared the tyrannosaur’s anatomy with that of Ornithosuchus.

When I look at the old restorations (including skeletal ones) most dinosaurs tend to look “sluggish”, with the dragging tail, you end up with a “stunned kangaroo” look. This sort of thing is one of the few times when post-modernism actually works (i.e. different ways of looking at the same evidence, influenced by different paradigms.) Mantell initially reconstructed Iguanodon with the proportions of a lizard, and these days we’re disappointed if a maniraptoran doesn’t get feathers. Charles Knight et al fall somewhere between these two extremes. (OK I know one is closer to the truth than the other two, but that’s not my point).

Glacier Bay, Alaska, is known as a place where humpback whales are often observed sleeping. They just float around like logs. I have a photo somewhere, but not much can be made out.

I saw a giraffe sleeping on its side once, in South Luangwa Nat’l Park, Zambia (another one was standing by).

As for small birds, sleeping preferences differ a lot between species. Grouse dive in the snow to sleep in winter, kingfishers always sleep on exposed low branches, etc. It’s been speculated that some tits also sleep under the snow on very cold nights, but no solid evidence exists, as far as I know.

It’s been speculated that some tits also sleep under the snow on very cold nights, but no solid evidence exists, as far as I know.

It’s not only parids that do it; there are quite a few small passerine species (such as buntings, finches, and goldcrests) that have been reported to dig holes into the snow, and to spend the night in these – see, e.g., Bagg (1943), Sulkava (1969), Novikov (1972), Lagerstr├Âm (1979), Helle (1980) and Pruitt (2005).

While this phenomenon thus seems to be rather common, there do not seem to be many published photographs of the actual act of passerines digging into the snow. But after a bit of searching, I did manage to find this Finnish ornithology blog. The bird in the pictures is an Arctic redpoll Carduelis hornemanni which was photographed late in the afternoon while it was busy digging in the snow – presumably with the intent of spending the night there (the temperature at the time was -25┬░ C). For some reason, it gave up and flew away shortly after the photos were taken, however. (Incidentally, according to the observer the digging was mostly done with the wings.)

At least this one seems to, according to the description. (Of course, snow consistency probably makes a big difference; I’d expect such digging to be relatively easy for the bird when the conditions are very cold and the snow is ‘dry’.)